Silicon force sensor and method of using the same

ABSTRACT

The various technologies presented herein relate to a sensor for measurement of high forces and/or high load shock rate(s), whereby the sensor utilizes silicon as the sensing element. A plate of Si can have a thinned region formed therein on which can be formed a number of traces operating as a Wheatstone bridge. The brittle Si can be incorporated into a layered structure comprising ductile and/or compliant materials. The sensor can have a washer-like configuration which can be incorporated into a nut and bolt configuration, whereby tightening of the nut and bolt can facilitate application of a compressive preload upon the sensor. Upon application of an impact load on the bolt, the compressive load on the sensor can be reduced (e.g., moves towards zero-load), however the magnitude of the preload can be such that the load on the sensor does not translate to tensile stress being applied to the sensor.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of prior U.S. application Ser. No.14/064,573, filed Oct. 28, 2013, which is incorporated herein byreference in its entirety.

STATEMENT OF GOVERNMENTAL INTEREST

This invention was developed under contract DE-AC04-94AL85000 betweenSandia Corporation and the U.S. Department of Energy. The U.S.Government has certain rights in this invention.

BACKGROUND

Designing sensors that can measure and survive shock loads in extremelyharsh real-world environments, while still providing a high signal tonoise (S/N) ratio and significant bandwidth, can be difficult. Further,a small sensor form factor (e.g. a microelectromechanical systems (MEMS)sensor) can be necessary to prevent the sensor deleteriously affectingthe structure under load, while facilitating arrayed and embeddedapplications. Possible applications for a very small sensor that cansurvive high shock loads and provide a high S/N ratio with a highbandwidth can include crash-test dummy instrumentation, impactforensics, embedded sensors in smart surfaces for lifetime structuralhealth monitoring, etc. Such sensors can indicate when a structuralelement (e.g., an aircraft wing, a bridge pylon, etc.) has experiencedan out-of-bounds load due to an extreme event (e.g., an explosion, anearthquake, etc.). A small sensor can also find application measuring asystem response to impact loads, as well as for validating modeling andsimulation tools.

There are three main types of normal load (pressure) sensors:capacitive, piezoelectric, and piezoresistive. Each type has performanceand manufacturing advantages and shortcomings associated therewith.Capacitive sensors typically have a very small signal (e.g., change incapacitance) relative to the overall sensor capacitance associated withthe complete circuit. Accordingly, a parasitic capacitance associatedwith signal lines on a chip separated from a chip ground by a thin oxidelayer can be difficult to avoid as the ground is intimately connected tothe sensing element. Such factors tend to reduce the signal to noiseratio for a capacitive sensor. Further, tight tolerances on the smallsensing capacitor gap can also be problematic as a very small gap isdesirable to achieve higher sensitivity, however a very small gap canlimit a range of gap deformation and further lead to an increase insensor response signal variability as a function of dimensionalvariability of the gap.

Piezoelectric sensing elements are typically quartz crystals whichrequire poling and assembly, including bonding of the crystal into anelectronic package. Recent advances in the fabrication of piezoelectricthin films are facilitating monolithic fabrication of piezoelectricsensors with a reduction in the reliance on adhesives at the criticalsensing element level. An advantage of piezoelectric sensors is thatthey can generate an output voltage with no input voltage required,although an input voltage is required for any in-package signalamplification or processing electronics. However, piezoelectric sensorslack a robustness required to survive dynamic shock loads of a highmagnitude.

Typically a DC signal (e.g., a low frequency response) is not sensed ina piezoelectric sensor, while it can be for a piezoresistive sensor.Additionally, monolithic fabrication techniques for piezoresistivesilicon (Si) sensing elements are well developed. Si has a largepiezoresistive gage factor, can be mass-fabricated, has a high elasticmodulus and possesses a high ultimate strength in uniaxial compression.However, Si is a brittle material with tensile and flexural strengthsbeing much lower in comparison with its compressive strengthperformance. Such material properties can render measurement of highlydynamic loads problematic with a Si sensor material. An applicationwhich can be particularly problematic is where dynamic loading caninduce tensile stress waves into structural components which can lead tothe generation of microcracking in the Si sensor material.

SUMMARY

The various, exemplary, non-limiting embodiments presented herein relateto a sensor which can measure and survive shock loading while stillproviding a high signal/noise ratio with significant bandwidth arepresented. In an exemplary, non-limiting embodiment, a load sensingsystem can comprise a load sensor comprising a piezoresistive sensor.The load sensing system can further comprise a load applicationcomponent configured to locate the load sensor on a supportingstructure. In a further embodiment, the load application component canapply a compressive preload to the load sensor, wherein the compressivepreload results in application of a compressive load to thepiezoresistive sensor. And in a further embodiment, while under anoperational load, the load application component can effect a relaxationin the compressive load on the piezoresistive sensor.

A further exemplary, non-limiting embodiment comprises a method formeasuring a load while providing a high signal/noise ratio withsignificant bandwidth. The method can comprise applying a compressivepreload to a piezoresistive device, wherein the compressive preload isgreater in magnitude than an operating range of operational loads. Themethod can further comprise transmitting an operational load within theoperating range to the piezoresistive device so as to produce areduction in the compressive preload. The method can further comprisemeasuring the reduction in the compressive preload to facilitatedetermining a magnitude of the operational load.

A further exemplary, non-limiting embodiment comprises a load sensingsystem, wherein the system can comprise a load sensor comprising atleast one piezoresistive sensor, a first washer, and a load applicationcomponent. In an embodiment, the load application component can comprisea nut and bolt, the bolt comprising a bolthead, a boltshaft and athreaded portion of the boltshaft. The load application component can beconfigured to locate the load sensor and the first washer on asupporting structure, wherein the bolt is located in a hole extendingfrom an outer surface of the supporting structure to an inner surface ofthe supporting structure. The bolt can be located such that the boltheadis situated proximal to the outer surface and the boltshaft extendsthrough the hole beyond the inner surface. In an embodiment, the firstwasher can be situated between the bolthead and the outer surface. Theload application component can be further configured to apply acompressive preload to the load sensor by tightening the nut on theboltshaft, resulting in application of a compressive load to the atleast one piezoresistive sensor. In an embodiment, the compressive loadcan generate a first voltage state at the at least one piezoresistivesensor. The load application component can be further configured toreceive an operational load on the bolthead. The operational load cancause a compression of the washer and the operational load can betransferred along the boltshaft causing a relaxation in the compressivepreload on the load sensor. In an embodiment, the relaxed compressiveload can generate a second voltage state at the at least onepiezoresistive sensor.

The above summary presents a simplified summary in order to provide abasic understanding of some aspects of the systems and/or methodsdiscussed herein. This summary is not an extensive overview of thesystems and/or methods discussed herein. It is not intended to identifykey/critical elements or to delineate the scope of such systems and/ormethods. Its sole purpose is to present some concepts in a simplifiedform as a prelude to the more detailed description that is presentedlater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a sensor system, according to anembodiment.

FIG. 2 is three dimensional view of a sensor system according to anembodiment.

FIG. 3 is a schematic of a sensing element, according to an embodiment.

FIG. 4 is a sectional view of a sensing element, according to anembodiment.

FIG. 5 is a sectional view of a sensor assembly which includes a sensingelement, according to an embodiment.

FIG. 6 a photograph illustrating a sensor assembly which includes aplurality of sensing elements, according to an embodiment.

FIG. 7 is a photograph illustrating a side view of a sensing elementwith an etched cavity, according to an embodiment.

FIG. 8 is a photograph illustrating a sensing element, with a tracelocated thereon, according to an embodiment.

FIG. 9 illustrates a die mount to locate a sensor assembly, according toan embodiment.

FIG. 10 is a block diagram illustrating a sensor assembly with a portionof the sensor assembly protruding out of a die mount opening, accordingto an embodiment.

FIG. 11 is a photograph illustrating a load sensor assembly, accordingto an embodiment.

FIG. 12 is a FEM study illustrating strain distribution in a sensingelement, according to an embodiment.

FIG. 13 is a FEM study illustrating strain distribution in a sensingelement, according to an embodiment.

FIG. 14 presents a chart depicting the effect of fillet radii onfracture load, according to an embodiment.

FIG. 15 presents a chart representing testing data for quasi-staticcompressive loads for a number of sensing elements, according to anembodiment.

FIG. 16 presents a chart representing testing data for quasi-staticcompressive loads for a number of sensing elements, according to anembodiment.

FIG. 17 presents a calibration curve for a sensor, according to anembodiment.

FIG. 18 presents a calibration curve for a sensor, according to anembodiment.

FIG. 19 presents a chart illustrating measured forces for a square wavepulse input, according to an embodiment.

FIG. 20 presents a chart illustrating measured forces for a trianglewave pulse input, according to an embodiment.

FIG. 21 presents a chart for a triangle shaped pulse with an appliedpreload, according to an embodiment.

FIG. 22 is a sectional view of a sensor system, according to anembodiment.

FIG. 23 is a flow diagram illustrating an exemplary, non-limitingembodiment to facilitate measurement of an impact load based upon arelaxation of a compressive load.

FIG. 24 is a flow diagram illustrating an exemplary, non-limitingembodiment to fabricate a load sensing component.

DETAILED DESCRIPTION

Various technologies pertaining to a sensor which can measure andsurvive shock loading while still providing a high S/N ratio with asignificant bandwidth are presented, wherein like reference numerals areused to refer to like elements throughout. In the following description,for purposes of explanation, numerous specific details are set forth inorder to provide a thorough understanding of one or more aspects. It maybe evident, however, that such aspect(s) may be practiced without thesespecific details. In other instances, well-known structures and devicesare shown in block diagram form in order to facilitate describing one ormore aspects.

Further, the term “or” is intended to mean an inclusive “or” rather thanan exclusive “or”. That is, unless specified otherwise, or clear fromthe context, the phrase “X employs A or B” is intended to mean any ofthe natural inclusive permutations. That is, the phrase “X employs A orB” is satisfied by any of the following instances: X employs A; Xemploys B; or X employs both A and B. In addition, the articles “a” and“an” as used in this application and the appended claims shouldgenerally be construed to mean “one or more” unless specified otherwiseor clear from the context to be directed to a singular form.Additionally, as used herein, the term “exemplary” is intended to meanserving as an illustration or example of something, and is not intendedto indicate a preference.

The various embodiments presented herein relate to a sensor configuredto measure high magnitude forces over a wide bandwidth, where suchforces can range from static loading through repetitive loadingextending to frequencies of several hundred kilohertz (kHz) or more. Inan embodiment, the sensor can be configured as a washer-like device andcan include at least one embedded piezoresistive sensing element (e.g.,a MEMS based sensor). In a further embodiment, the sensing element canbe formed from Si. A Si sensing element can be fabricated to include aphotolithographically patterned strain gage(s) on a deformable senseplate to provide a high sensitivity linear response (e.g., about 1-10mV/1000 lbf, i.e., about 0.2-2 mV/kN) to input loads of up to about10,000 lbf (about 45000 N). The patterned strain gages, or ‘wires’facilitate operation of the sensor as a bridge circuit to generate ameasurable voltage difference. As described herein, a range of sensorscan be fabricated to operate over a range of applied loads byimplementing relatively simple design changes, for example, changing thediameter of a sense plate region, changing the thickness of a senseplate region, etc. Small Si sensing elements (e.g., of about 10 μm toabout 100 μm in diameter) enable multiple sensors to be embedded in awasher-like device to facilitate redundant load measurement and furtherenable a determination of loading direction (e.g., off-axis, angled orside loads) by comparing outputs from sensors at different locations onthe washer.

Readily available non-brittle engineering materials having ductileand/or compliant properties can be utilized to package the Si sensordie(s) in a layered material stack arrangement that can protect thebrittle Si to facilitate survival of impact loads of a high magnitude(s)For example, the enclosing materials can act to dampen undesirable highfrequency signals that may cause damage to the Si and associatedcomponents. Such materials can include glass-reinforced epoxy laminatesheets (e.g., a printed circuit board, PCB), copper (Cu), polyimide,aluminum (Al), steel, etc. A low-level (micro-) compressive preload canbe applied to the sensor to ensure that any gaps in the material stackare closed. Prior to operation a high level (macro-) compressive preloadcan be applied the sensor system including the Si sensing element. Themagnitude of the macro-preload may be selected, e.g., such thatirrespective of the operational loading on the sensor system, the Sisensing element will always experience compressive loading.

The sensor design(s) facilitates control of a stress level at the Sisensor die and thus the effective load range of the sensor, whilemaintaining a high S/N ratio (e.g., of greater than about 100:1).Further, the various sensor designs presented herein are amenable toscalable manufacturing, take advantage of the high sensitivity and highcompressive strength of Si, and they facilitate integration of thesensor with other sensors and actuators as part of a complete impactresponse system. Further, owing to application of a piezoresistivesensor, sensor response is typically linear (e.g., as compared with aquadratic response for a capacitive sensor), whereby the linear responsecan ease calibration and electronic signal conditioning.

FIGS. 1 and 2 illustrate a load sensing apparatus 100 in accordance withvarious non-limiting embodiments, where FIG. 1 illustrates a verticalsection through A-A and FIG. 2 presents a three dimensional (3D)representation. As shown in FIGS. 1 and 2, a supporting structure suchas a bolt 130 and nut 140 can be utilized to locate a load sensor 120 ona housing 110. In an aspect, the housing 110 can be a wall, a panel, askin, a frame, or other structure of a device into which the load sensor120 can be incorporated.

As also shown in FIG. 1, at least one sensing element 122 can be locatedinside the load sensor 120, with design and operation of the sensingelement(s) 122 described further herein, particularly with reference toFIGS. 3, 4, and 5. The bolt 130 comprises a bolthead 131, a boltshaft133 and a threaded portion 135 of the boltshaft 133 onto which the nut140 can be attached (e.g., nut 140 can be threaded onto the boltshaft133). Bolt 130 is located in a hole 116 formed (e.g., drilled) in thehousing 110, with the boltshaft 133 and the threaded portion 135 locatedin the hole 116. The bolthead 131 is located in a recess 115 formed inthe housing 110 about the hole 116 (e.g., recess 115 is a countersunkhole). In an embodiment, the bolthead 131 can be situated proximal to anouter surface (or sidewall) 118 such that a load (e.g., P_(applied) asfurther described herein) applied to the outer surface 118 also acts, inpart, on the bolthead 131. The base of the recess 115 acts to retain thebolthead 131 while the boltshaft 133 extends through the hole 116 andprotrudes from an inner sidewall (or surface) 117 of the housing 110opposite to the recess 115. Load sensor 120 having a washer-likeconfiguration can be located on boltshaft 132. Nut 140 can be utilizedto locate the sensor 120 against the inner sidewall 117. Such locatingof sensor 120 against the inner sidewall 117 by nut 140 may beperformed, e.g., by tightening nut 140. The tightening of nut 140 canfurther act to apply tension T on bolt 130 which locates bolthead 131 inrecess 115. Electrical connection to the sensing elements 122 can be viaconnector 180, with signals (e.g., a change in voltage applied to the atleast one sensing element 122) being transmitted and/or received by loadmeasuring system 182. Based on electrical changes at one or more sensingelements such as element 122, a state of the at least one sensingelement 122 can be determined, such as the difference between a firststate (e.g., a preloaded state) and a second state (e.g., under impactloading). For example, an impact load can cause a relaxation in thepreloaded state of a load sensor 120 (as further described below) andthe one or more sensing elements, such as element 122, can include aWheatstone bridge, whereby the change from the first loading state tothe second loading state causes a change in voltage across theWheatstone bridge. The Wheatstone bridge facilitates measurement of theload on a sensing element 122 by balancing the resistance in two legs ofthe bridge circuit.

It is to be appreciated that the term ‘locate’ is utilized herein toconvey positioning and/or attachment of a component, e.g., a firstcomponent relative to a second component. For example, the firstcomponent can be located adjacent to the second component. In anotherexample, the first component can be located with reference to the secondcomponent, hence, while the first component and second component may notbe adjacent or touching, the second component (e.g., load sensor 120)may be located with reference to a first component (e.g., any of aninner sidewall 117, recess 115, hole 116, etc.), where the secondcomponent is separated from the first component by a third component(e.g., any of washers 150, 160, or 170 as further described below).

In an embodiment, rather than placing the load sensor 120 under directcompressive loading (e.g., the load sensor is located between the nut140 and the housing 110, as shown, e.g., in FIG. 22), a plurality ofgapping components, e.g., washers 150, 160 and 170 can be utilized toapply a compressive preload, P_(preload), to the load sensor 120.Application of a preload can facilitate linearization of the response ofload sensor 120 as a function of a reduction in the compressive loadingof the sensing element(s) 122 located inside the load sensor 120, asfurther described herein. In an embodiment, washer 150 can be located,i.e., situated, in the recess 115 between the bolthead 131 and thebottom of the recess 115. Washers 160 and 170 can be located onboltshaft 133, e.g., washer 160 can be located between the load sensor120 and sidewall 117, while washer 170 can be located between the loadsensor 120 and the nut 140, as illustrated in FIG. 1.

Washers 150, 160 and 170 can be formed from a material having a ductileand/or compliant nature such that during application of either ofP_(preload) or P_(applied), a load (e.g., L₁ at washer 160 and L₂ atwasher 170) is maintained on load sensor 120, where such materialsinclude, in a non-exhaustive list, Cu, Al, polyimide, steel, PCB, etc.Washers 150, 160 and 170 can be formed from a resilient material, perthe above, which can have a degree of elastic recovery such that as aload is intermittently placed on any of washers 150, 160 or 170, thewasher can be compressed and upon removal of the intermittent load thewasher can expand back to, or near to, an original size, therebycompensating for the various dimensional changes in structure 100 as theload is applied and removed.

A high degree of damping can increase a load survivable by load sensor120, but typically (e.g., with a conventional load sensor) such dampingcan lead to a reduction in the bandwidth, where bandwidth herein relatesto the frequency range and/or highest frequency being measured duringloading. Accordingly, the configuration illustrated in FIGS. 1 and 2facilitates protection of the load sensor 120, whereby any of thewashers 150, 160 and/or 170, in conjunction with the bolt 130 and nut140, can act to absorb a damaging portion of an impact load P_(applied)while imparting a known portion of the load P_(applied) through the loadsensor 120 (and accordingly any sensing element(s) 122 includedtherein).

In an embodiment, a macroscopic preload can be applied to the loadsensor 120 (and accordingly any sensing element(s) 122 included therein)based upon tightening of nut 140 along bolt thread 135. In anotherembodiment, a microscopic preload can be applied to a sensing element122 based upon fabrication of the load sensor 120, as further describedherein.

During preloading, P_(preload), of the load sensor 120, tightening ofnut 140 along bolt thread 135 can place the load sensor 120 and washers150-170 under compression loading C, and accordingly place the bolt 130in a state of preload tension T, with the compressive and tensileloading being respectively applied by the shoulder 132 of bolt 130 andworking face 142 of nut 140. During operation, application of an impactload, P_(applied), on the exposed upper surface 134 of bolthead 131,e.g., in direction F, can act to reduce the preload tension T in thebolt 130 and thereby reduce the P_(preload) compression C on the loadsensor 120. In an embodiment, if a transmitted portion of the impactload P_(applied) does not exceed the P_(preload) compression C appliedto the load sensor 120, the load sensor 120 (and accordingly any sensingelement(s) 122 included therein) can survive application of impact loadP_(applied) as the load sensor 120 has already withstood the morecontrolled (e.g., low loading rate) application of P_(preload).

For example, sensing element(s) 122 can always be in a state ofcompression, or near compression, where in an example ifP_(preload)=10000N, and P_(applied)=4700N, the sensing element 122 isstill experiencing a compressive load of 5300N. Furthermore, in anotherembodiment, the application of P_(preload) can also ensure that the loadsensor 120 is in a state of compression of sufficient magnitude tofacilitate linear response of the load sensor 120 during application ofthe impact load P_(applied). Effectively, application of the P_(preload)can remove any gaps or looseness in the hardware stack (e.g., components110, 120, 130, 140, 150, 160, and 170, and also components included insensor 120 as described below) resulting in a tight component assemblythat can smoothly transmit a portion of the impact load P_(applied) tothe load sensor 120 (and accordingly any sensing element(s) 122 includedtherein). In an aspect, it can be considered that there are two levelsof protection, the microscopic compressive preload applied to the sensor120 and sensing element(s) 122 located in load sensor 120, and themacroscopic compressive P_(preload) applied to the load sensor 120 andthe portion of the P_(preload) acting to maintain the hardware stacktightly coupled.

In an embodiment, one or more sensing elements 122 can be incorporatedinto a load sensor 120 to facilitate load measurement as previouslydescribed with reference to FIGS. 1 and 2. FIGS. 3 and 4 illustrate adesign for a sensing element 300 (which can be considered to beequivalent to sensing element 122), according to an embodiment, whereFIG. 4 is a sectional view along B-B. The sensing element 300 can beformed from a plate 310 of material, which may, e.g., be single crystalSi.

In an example, the thickness of plate 310 can be on the order of about100 μm thick to facilitate high load measurement. Further, the thicknessof the plate 310, or a portion thereof, can be thinned (e.g., to athickness of about 10 μm) by any suitable technique such as an etchprocess (e.g., a Bosch etch), where the broken line 313 on FIG. 3indicates the location of the sidewalls 313 of a thinned region 312 onFIG. 4.

Alternatively, plate 310 can be constructed using silicon on insulator(SOI) device technology, whereby a wafer can be obtained of a requiredthickness with and/or without the thinned region 312. A surface (e.g., atop, or upper, surface in the plane of the illustration of FIG. 3) ofplate 310 can be patterned with at least one trace, e.g., traces 320and/or 330, as shown in FIG. 4. As presented further herein, traces 320and 330 can be formed from four separate traces 320A and 320B, and 330Aand 330B, where traces 320A and 320B are located relative to theperiphery of thinned region 312, and traces 330A and 330B can be locatedmore centrally relative to the thinned region 312.

As explained further herein, when a load (e.g., P_(preload) orP_(applied)) is applied to and/or distributed across the top surface ofthe plate 310, plate 310 can become deformed (e.g., by bending) aboutthe thinned region 312. Accordingly, deformation of plate 310 can leadto strain transduction occurring in the traces 320 and 330 on the topsurface of the plate 310, where in where in typical configurations thehighest strain tends to occur. While the traces 320 and 330 can be ofany dimension and form, in an embodiment the traces 320 and 330 can beformed from any suitable conductor such as doped (e.g., greater thanabout 1×10²⁶ cm⁻³) n-type polysilicon and can be of any suitabledimension, e.g., about 0.3 μm, in width and height. In a furtherembodiment, traces 320 and/or 330 can be isotropic. In anotherembodiment, traces 320 and/or 330 can have a piezoresistive gage factorcoefficient of about −18.

As shown in FIG. 3, the traces 320A, 320B, 330A and 330B can form aserpentine pattern repeatedly crossing back and forth over the thinnedregion 312 of the plate 310 that can experience the highest radialstrain during deformation of the plate 310.

In an embodiment, the traces 320A, 320B, 330A and 330B can be arrangedas a full Wheatstone bridge, in which a plurality of bridge resistanceconnections 340-370 are collectively patterned on the surface of plate310. Patterning the plurality of bridge resistance connections 340-370onto the surface of plate 310 can mitigate any temperature variationeffects as all the resistive legs (e.g., traces 320A, 320B, 330A and330B) of the Wheatstone bridge can be subject to the same temperatureand thermal boundary conditions. Further, patterning the plurality ofbridge resistances 340-370 onto the surface of plate 310 can alsomaximize sensitivity as the opposing resistor pairs 340 & 350 and 360 &370 in the Wheatstone bridge can be subject to either tension (outerresistors 340 and 370 located at the edge of plate 310) or compression(inner resistors 350 and 360 located at the center of plate 310).Patterning the plurality of bridge resistances can cause differences inresistance due to strain to increase the change in voltage, rather thanoffset it. In embodiments, the Wheatstone bridge does need to bebalanced. Obviating such a need can facilitate monitoring the offset(e.g., at outputs 340 and 370) for problems associated with electricaldesign, circuit connections, input/output electrical connections, andthe like. It is to be appreciated that any suitable resistance can beutilized to facilitate operation and sensing by the Wheatstone bridge(e.g., traces 320A, 320B, 330A and 330B), in an embodiment, a resistanceof each leg of the bridge can typically be in the order of tens ofkilohms.

While not shown in FIGS. 3 and 4, a layer of insulating film (e.g.,silicon dioxide (SiO₂), and/or silicon nitride (SiN) layer 570 shown inFIG. 5) can be deposited between the traces 320 and 330 and the surfaceof plate 310 to prevent shorting. Further, while not shown in FIGS. 3and 4, a layer of insulating film (e.g., layer 580 in FIG. 5) can alsobe applied to cover the surface of the traces 320 and 330 to provide ameasure of wear protection.

As previously mentioned, the brittle sensing element 300 can be packagedto protect the sensing element from the most extreme loads and canfurther transmit a percentage of the applied load to the die 310.Protection afforded by sensor packaging enables the sensor element 300survive the high loads and loading rates. FIGS. 5-11 provideillustration of various packaging embodiments and a testingconfiguration, and are presented to facilitate understanding ofcalibration of an applied load to a measured sensor response. Basedthereon, various predictions can be made regarding a transfer functionthrough a sensor package for an anticipated load level(s) for thevarious materials, stiffness, and thicknesses chosen in the packagedesign. The various calibration curves, applied loads, FEM analyses,sensor dimensioning, material selection, etc., are presented tofacilitate understanding of the various concepts presented herein andare not to be taken as definitive values, with any parameter magnitude,material, dimension, etc., being replaceable by any other value suitablein accordance with the embodiments presented herein. Hence, theparameter magnitude(s), material(s), dimension(s), etc., can be selectedto facilitate design of a sensor, and adjusted for different anticipatedloads in different sensor applications.

FIG. 5 illustrates a sensor assembly 500 which includes a sensingelement 300, according to an embodiment. As illustrated, a plate 310 haslocated thereon traces 320 and 330, as previously described in FIGS. 3and 4. Thinned region 312 indicates where material has been removed(e.g., by Bosch etching) to facilitate thinning of plate 310 to athickness R which is less than the original thickness P of plate 310.

In an embodiment, the thinned plate region 312 can increase the degreeof deformation which can occur at that region relative to a plate with aconstant thickness P throughout.

Electrical connection to traces 320 and/or 330 is facilitated byelectrical contacts 510 which can be connected to a printed circuitboard (PCB) 530 via electrical contacts 520 and 590, bond pads 550 andtrace elements 560. In an embodiment, the bond pads 550 can be formedfrom any suitable material, e.g., aluminum. In an embodiment, traceelements 560 can be formed on the PCB 530, and further can be formedfrom any suitable conductive material, e.g., gold. In an embodiment,electrical contacts 590 can be solder bumps and in a further embodiment,electrical contacts 520 can be a die attach, formed, for example, fromindium (In). As previously mentioned, traces 320 and/or 330 can beoptionally located between a lower insulating layer 570 and/or an upperinsulating layer 580. Further, an adhesive layer 540 can be locatedbetween the upper layer 580 and the PCB 530, where adhesive layer 540can be formed from any suitable material such as an epoxy resin.

FIG. 6 is a photograph illustrating a sensor assembly 600 which includesa plurality of sensor assemblies 500 (and according sensing elements300), according to an embodiment. As previously mentioned, the exampleconfiguration presented herein relates to a load sensor 120 beinglocated in a compressive loading system comprising a bolt 130 and a nut140.

Accordingly, as shown in the example configuration, a plurality ofsensing elements 300 (e.g., comparable to sensing elements 122 ofFIG. 1) can be located on a PCB 530, where in the example embodiment,the PCB 530 is in the form of a washer-like configuration to facilitatelocation of the load sensor 120 on to the boltshaft 133. In the exampleconfiguration presented in FIG. 6, four sensing elements 300 (eachlocated in a sensor assembly 500) are located on PCB 530, wherebyreadings from each of the four sensing elements 300 can be analyzed and,based thereon, a determination can be made regarding a direction ofloading relative to the operating plane of the four sensing elements300. PCB 530 has further formed thereon a plurality of traces 610 whichare connected to respective inputs and/or outputs 340-370.

FIG. 7 is a photograph illustrating a side view of a sensing element 300with the etched cavity 312 readily discernible, according to anembodiment. As shown, each sensing element 300 can be located on PCB 530by an adhesive layer 540, where during location of a sensing element 300on to PCB 530 the adhesive layer 540 can wick into a gap formed betweenthe electrical contacts 520 on PBC 530 and the upper oxide layer 580.FIG. 8 is a photograph illustrating a plate 310, with a trace 320 and/or330 located thereon, according to an embodiment. Further indicated onFIG. 8 are the bridge resistance connections 340-370. Electricalcontacts 520 can be utilized to supply and/or measure voltage, etc., astraces 320 and/or 330 undergo loading deformation. Further, additionalindium bumps 810 can also be utilized as symmetrical support forelectrical contacts 520.

FIGS. 9, 10 and 11 illustrate example configurations for packaging of asensor, according to an embodiment. As previously mentioned, the variousembodiments presented herein can be directed towards incorporation ofone or more sensing elements (e.g., sensing elements 300) into awasher-like configuration. As shown on FIG. 9, a sensor assembly (e.g.,sensor assembly 500) can be incorporated into a die mount 910. Die mount910 can include a plurality of openings 920 (e.g., well cutouts) wherebyeach opening can be utilized to locate a sensor assembly 500. As shownin FIG. 10, each sensor assembly 500 can be located in the die mountopening 920 such that a portion of the sensor assembly 500 can protrude,by distance M, above the surface H of the die mount 910. Hence, duringassembly of a washer assembly (e.g., assembly 1110 shown in FIG. 11), awasher or disc of material 1010 can be placed on the exposed surface ofthe sensor assembly 500, such that a micro-level preload can be appliedto the sensing elements 300 in the sensor assembly 500 which can furthercause the washer 1010 to deform over the surface of sensing element 300by the distance M. In an embodiment, washer 1010 can be formed of acompliant material such as nylon. A second washer 1020 can be placedunder the PCB 530 as required to facilitate protection of the sensingelement 310.

It is to be appreciated that any number of layers can be utilized duringassembly and packaging of a sensing element 300. Hence, once the finalwasher assembly 1110 is assembled with the sensor assembly 500 andwashers 1010 and 1020 located in an outer shell 1120, the sensorassembly 500 and the respective sensing elements 500 can have amicro-level preload applied thereto. During subsequent assembly of thewasher assembly 1110 (e.g., forming load sensor 120) into the loadsensing device apparatus 100, a further compressive load (e.g., amacro-level preload) can be applied to the sensor assembly 500, wherebythe macro-level preload and the micro-level preload can combine to formP_(preload).

To facilitate understanding, the following presents calculations andtest data relating to a structure (e.g., sensor assembly 500) fabricatedin accordance with the various embodiments presented herein. In anembodiment, a primary change in resistance across the Wheatstone bridge(e.g., traces 320A, 320B, 330A and 330B) can be due to strain beinginduced at the upper surface of the plate 310. The strain can be inducedas a function of the plate 310 being deformed, e.g., bent. In anembodiment, the strain of the upper surface of plate 310 can bemaximized at edge region E (e.g., as shown in FIG. 3), where thethinning of region 312 from a thickness P (e.g., thickness of plate 310)to a thickness R, can concentrate strain at region E.

Finite element (FEM) studies showing the strain distribution in plate310, and traces 320 and 330, are presented in FIGS. 12 and 13.

Optical inspection of the thinned region 312 formed by the etchingprocess indicate that the fillet radius (FR) region where the etchedplate meets the remainder of the plate 310 was not sharp, but rather hada significant fillet radius estimated at 100 to 200 μm. The effect offillet radius is further address in FIG. 21.

It is to be appreciated that the solutions can over-predict edgestresses. A load P_(applied) was applied to the sensor in thesecalculations rather than the fraction of the load that actually is feltby the sensor due to load division between the sensor 310 and thepackage 910. It is to be noted that the calculation identifies regionsof high stress near the edges of the plate in FIG. 12, however such aconcentration can be mitigated with a larger fillet. In this solution adistributed load is applied across a quadrant of a circular sensor plateregion 312 surrounded by a Si support structure 310. A nylon washer(soft material) covered by a steel washer (stiff material) can bemodeled on top of the Si plate (neither the nylon nor steel elements areshown) to incorporate a deforming element and maintain load elementcontact with the plate 310 as it deforms.

As previously mentioned, the washers (e.g, washers 1010 and 1020) can beconsidered as a first level of packaging protection for the sensorelement 310. The sensor plate 310 can fail (fracture at the edge of theplate) at the loads modeled in FIGS. 12 and 13 due to the relatively lowstrength of Si in bending, further presented in the design calculationshereinafter. It is to be noted that forming a sensor plate with SOIcould result in a sharper FR and hence, may be more closely modeled inaccord with FIGS. 12 and 13.

FIG. 14 presents chart 1400 illustrating the effect of fillet radii onfracture load, e.g., in the corner of thinned region 312, according toan embodiment. FEM analysis of fracture load based on maximum stress atmembrane radius (plate edge) for different fillet radii are shown, plot1410 is the determined values for a 50 μm radius, plot 1420 is thedetermined values for a 100 μm radius, and plot 1430 is the determinedvalues for a 200 μm radius. A calculated fracture load from the exampledesign calculations presented herein is shown in line 1440, while themeasured fracture load is shown in line 1450 (note: Si sensor elementfractured at load just beyond that shown in FIG. 20).

Further, an approximate fracture strength for Si in bending is shown in1460. The fracture strength of Si is reached at a low load (˜10000 N)for the small radius etch fillet of 50 μm (which is in approximateagreement with the design calculation herein) and at a much higher load(˜30000 N) for the large radius etch fillet of 200 μm (which is inapproximate agreement with an experimental fracture load, as presentedherein). Thus, the results shown in FIG. 12 support utilization of alarge etch fillet radius (estimated at about 200 μm) to reduce thestress at the plate edge, increasing the maximum survivable load anddecreasing the sensor signal by an amount consistent with the observedexperimental results.

To facilitate understanding of the various embodiments presented herein,the following equations are presented. As an approximation, the plate310 can be considered, in aspect, as a clamped circular plate having afixed/fixed boundary at the edge E. An analytical expression for maximumstress (σ_(R)) at the edge of a clamped circular plate is presented inEqn. 1:

$\begin{matrix}{\sigma_{R} = \frac{3\pi\; r^{2}w}{4\;\pi\; t^{2}}} & {{Eqn}.\mspace{14mu} 1}\end{matrix}$

Further, the associated strain (ε_(R)) can be calculated using Young'smodulus, according to Eqn. 2:

$\begin{matrix}{\varepsilon_{R} = {\frac{\sigma_{R}}{E} = \frac{3\; r^{2}w}{4\; t^{2}E}}} & {{Eqn}.\mspace{14mu} 2}\end{matrix}$

where w is the distributed load (N/m²), E is Young's modulus (N/m²), tis the thickness of plate 312, and r is the plate radius (m). Owing toradial strain having a larger magnitude than a circumferential strain atthe edge of the circular plate (e.g., edge E of thinned region 312),Eqns. 1 and 2 can be considered to be directed towards radial strain.

Returning to Eqns. 1 and 2 and incorporating a piezoresistive gagefactor for highly doped n-type polysilicon (e.g., G=−18), a change inresistance (ΔR) for a given strain (ε_(R)) can be calculated, accordingto Eqn. 3:ΔR=GRε _(R)  Eqn. 3

Further, an approximate bridge circuit output voltage (V_(Out)) as afunction of input voltage (V_(in)) can be derived for a half bridgedesign according to Eqn. 4:

$\begin{matrix}{V_{out} = {2\;{V_{in}\left\lbrack \frac{R_{1} - R_{2}}{R_{1} + R_{2}} \right\rbrack}}} & {{Eqn}.\mspace{14mu} 4}\end{matrix}$

where R₁ is the resistance of the outer trace 320 (e.g., traces 320A and320B) and R₂ is the resistance of the inner trace 330 (e.g., traces 330Aand 330B), according to FIG. 3.

With reference to FIG. 3, for the sensor design the resistances of thetwo outer traces 320A and 320B can be made equal to each other and theresistances of the two inner traces 330A and 330B can be made equal toeach other. Accordingly, the sensor design can be approximated asWheatstone halfbridge circuits owing to the shorter center of the trace320 resistors, while under compressive loading, and hence can experienceless strain and contribute less change in resistance than the longeredge of plate resistance traces 330 may do.

For a simple circuit design, the input ground 350 and V_(output) 340and/or 370 can be connected to minimize noise, according to FIG. 3,which can effectively create a voltage divider circuit. For a voltagedivider circuit the change in output sense voltage for a given change inresistance (in this case due to strain at the edge of the plate(according to Eqn. 3)) can be derived according to Eqn. 5:

$\begin{matrix}{{\Delta\; V_{out}} = {\frac{R_{2}}{\left( {R_{1} + R_{2}} \right)^{2}}V_{in}\Delta\; R_{1}}} & {{Eqn}.\mspace{14mu} 5}\end{matrix}$

As previously mentioned, the materials in package 1110 surrounding thesensor assembly 500 can protect the sensing element(s) 300 from the mostextreme loads and can transmit a percentage of the load P_(applied) tothe sensing element(s) 300. All of this protection enables a sensorassembly 500 (and associated components) to survive high loading of ahigh frequency.

However, the relationship between applied load to a measured sensorresponse is required to be known, e.g., for calibration purposes.Accordingly, knowledge is required to be able to predict the transferfunction of the anticipated load levels (e.g., P_(applied) and/orP_(preload)) through the materials, stiffness, and thicknesses chosen inthe design of any of sensor assembly 500, sensing element(s) 300, washerassembly 1110, etc. Based on such knowledge, various design parameterscan be established and/or adjusted in accordance with the variousanticipated loads to be potentially encountered in a variety ofapplications.

With reference to the package configurations shown in FIGS. 5-11, thevarious components comprise a plurality of materials of differingproperties. The percentage of load transmitted to the sensing element(s)300 can depend on the stiffness of the materials forming the twoparallel load paths through the hardware stack: one through the sensorassembly 500 and one around the sensor assembly 500, as previouslydescribed with reference to FIG. 10. A primary deforming element is theepoxy washer 530, and the stiffness of each component in components 500,300, etc., can depend on the Young's modulus for each material (epoxy,steel, Si, etc.), the thickness of each component, and the area overwhich the load acts at each component.

At the interface(s) between the sensor assembly 500, the washer 1010 andand the support 910, a portion of the load is carried by the sensorassembly 500 and a portion by the surrounding support 910. The samedeformation occurs in the Si sensing element 300 as the steel in thesurrounding support 910, but a greater load is carried by the steel,primarily because it has much more area under load. The fraction of loadcarried by the sensing element 300 and that carried by the surroundingsteel structure 910 can be calculated according to:P _(App) =P _(SI) +P _(ST)  Eqn. 6

where P_(App) is the total load applied to the sensing element 300 andthe support 910, P_(SI) is the portion of the load passing through thesensing element 300, and P_(ST) is the portion of the load passingthrough the support 910. Ignoring any difference in height between thesensor assembly 500 (e.g., comprising sensing element 300) and thesupport 910, which would be eliminated by a preload procedure aspreviously described, it can be considered that the die and substrateare at the same height and subject to the same total applied load. Thesensor assembly 500 and the support 910 can deform the same amount (δ),but the stiffer structure will carry a larger proportion of the appliedload.F=kδ  Eqn. 7k=EA/L  Eqn. 8

$\begin{matrix}{\delta = {\frac{F}{k} = \frac{F \cdot L}{E \cdot A}}} & {{Eqn}.\mspace{14mu} 9}\end{matrix}$

In the preceding set of equations, F=P is the applied force or load, kis spring stiffness, E is Young's modulus, A is area, and L is length.Therefore, since δ is the same for the steel support 910 and the Siparts of the sensor assembly 500:

$\begin{matrix}{\delta_{ST} = {\delta_{SI} = {\left\lbrack \frac{P_{ST} \cdot L}{E_{ST}A_{ST}} \right\rbrack = \left\lbrack \frac{P_{SI} \cdot L}{E_{SI}A_{SI}} \right\rbrack}}} & {{Eqn}.\mspace{14mu} 10}\end{matrix}$

L is the same for the steel and Si and is equal to the thickness of thesensor assembly 500. Therefore:

$\begin{matrix}{P_{ST} = {P_{SI} \cdot \left\lbrack \frac{E_{ST}A_{ST}}{E_{SI}A_{SI}} \right\rbrack}} & {{Eqn}.\mspace{14mu} 11}\end{matrix}$

$\begin{matrix}{P_{APP} = {P_{SI} + {P_{SI}\left\lbrack \frac{E_{ST}A_{ST}}{E_{SI}A_{SI}} \right\rbrack}}} & {{Eqn}.\mspace{14mu} 12}\end{matrix}$

$\begin{matrix}{P_{SI} = {P_{APP}\left\{ \frac{E_{SI}A_{SI}}{{E_{SI}A_{SI}} + {E_{ST}A_{ST}}} \right\}}} & {{Eqn}.\mspace{14mu} 13}\end{matrix}$

$\begin{matrix}{P_{ST} = {P_{APP}\left\{ \frac{E_{ST}A_{SI}}{{E_{SI}A_{SI}} + {E_{ST}A_{ST}}} \right\}}} & {{Eqn}.\mspace{14mu} 14}\end{matrix}$

The fractional load P_(Si) applied to the Si, can be used to calculate asensor response change in output voltage, ΔV_(out) for a given appliedload. First, the distributed load on the sensor assembly 500 iscalculated, which is then utilized to calculate the strain at the edge Eof the sensor plate 310:

$\begin{matrix}{w = \frac{P_{SI}}{A_{SI}}} & {{Eqn}.\mspace{14mu} 15}\end{matrix}$

$\begin{matrix}{\varepsilon_{R} = {\frac{3\; r^{2}w}{4\; t^{2}E_{SI}} = {\frac{3\; r^{2}P_{SI}}{4\; t^{2}E_{SI}A_{SI}} = {\frac{3\; r^{2}P_{APP}}{4\; t^{2}}\left\lbrack \frac{1}{{E_{SI}A_{SI}} + {E_{ST}A_{ST}}} \right\rbrack}}}} & {{Eqn}.\mspace{14mu} 16}\end{matrix}$

$\begin{matrix}\begin{matrix}{{\Delta\; V_{out}} = {{\frac{R_{2}}{\left( {R_{1} + R_{2}} \right)^{2}}V_{in}\Delta\; R_{1}} = {\frac{R_{2}}{\left( {R_{1} + R_{2}} \right)^{2}}V_{in}{GR}_{1}\varepsilon_{R}}}} \\{= {{{\left\lbrack \frac{R_{1}R_{2}}{\left( {R_{1} + R_{2}} \right)^{2}} \right\rbrack\left\lbrack {V_{in}G} \right\rbrack}\left\lbrack \frac{3\; r^{2}P_{APP}}{4\; t^{2}} \right\rbrack}\left\lbrack \frac{1}{{E_{SI}A_{SI}} + {E_{ST}A_{ST}}} \right\rbrack}}\end{matrix} & {{Eqn}.\mspace{14mu} 17}\end{matrix}$

$\begin{matrix}{{\Delta\; V_{out}} = {\left\{ {{{\left\lbrack \frac{R_{1}R_{2}}{\left( {R_{1} + R_{2}} \right)^{2}} \right\rbrack\left\lbrack {V_{in}G} \right\rbrack}\left\lbrack \frac{3\; r^{2}}{4\; t^{2}} \right\rbrack}\left\lbrack \frac{1}{{E_{SI}A_{SI}} + {E_{ST}A_{ST}}} \right\rbrack} \right\} P_{APP}}} & {{Eqn}.\mspace{14mu} 18}\end{matrix}$

Based upon the previous equations and exemplary embodiments, thefollowing presents an exemplary determination for a maximum load thatcan be applied without sensor failure due to excessive stress at theedge E of the plate 310. The following further presents an exemplarymethod for measuring the sensitivity of sensor assembly 500 in terms ofvoltage change per unit input load.

The failure stress for the plate 310 in bending is approximately 500 MPaand the Young's modulus is 170 GPa. In an exemplary embodiment, theplate 310 can be 100 microns thick and 1 mm in diameter. With referenceto Eqns. 1 and 2 the distributed load that can cause the failure stressto be reached at the edge E of plate 310 can be determined to be:w=2.7×10⁷ N/m² (27 MPa, 3970 lbf/in²).

It is to be appreciated that w is the distributed load on the plate 310transmitted through the sensor assembly 500, not the load on the support910 which is to be determined. The associated maximum radial strain atthe edge E of the plate 310 can be calculated from Eqns. 1 and 2, andthe maximum change in resistance from Eqn. 3 for R₁=40 kOhm.

ε_(R)=0.0029, ΔR₁=2 kOhm

It is to be noted that either a thicker plate region 312 or a smallerdiameter plate can be utilized to increase the determined maximum load.However, this could result in a loss in sensor response signal, hence, adesign compromise can be made to determine the desired combination ofhigh maximum load and high sensitivity.

For the exemplary design presented herein (e.g., FIGS. 3-10), assumingfour sensing elements 300 included in sensor assembly 500 of size 2 mmby 1.4 mm:A _(Si)=4(2×10⁻³)(1.4×10⁻³)=1.1×10⁻⁵ m²

The steel area of support 910 is:A _(ST)=2.5×10⁻⁴−1.2×10⁻⁵=2.4×10⁻⁴ m²

for a 19.1 mm (0.75 inch) OD steel support washer 910 with a 6.35 mm(0.25 inch) diameter ID, and four cutouts 920 slightly larger than 2 by1.4 mm to accommodate the sensor assemblies 500:

E_(SI)=170 GPa and E_(ST)=200 GPa

Per the Eqns. 13 and 14 above:

P_(ST)=0.96P_(App) and P_(SI)=0.04P_(App)

For an evenly distributed load over the support washer 910 one fourth ofthis load is on each sensor assembly 500 (e.g., each of the four sensingelements 300), 0.01P_(App) per die. It was previously calculated that adistributed load of 2.7×107 N/m2 over the die will lead to sensor platefracture for a 100 μm thick, 1 mm diameter Si plate 310, accordingly:P _(SI-failure)=(2.7×10⁷)(1.1×10⁻⁵)=300N (68 lbf), across all foursensing elements 300.

P_(SI-failure,per die)=75N (18 lbf), per each sensing element 300.P _(App)=300/0.04=7500N (1700 lbf)

applied to the support washer 910 for sensor die failure. In terms ofthe normal stress on the full area of support washer 910 thiscorresponds to:

$\sigma_{washer} = {\frac{7500}{2.5 \times 10^{- 4}} = {30\mspace{14mu}{MPa}\mspace{14mu}\left( {4400\mspace{14mu}{lbf}\text{/}{in}^{2}} \right)}}$

The sensitivity of the output voltage to this applied load can becalculated from Eqn. 18:

ΔV_(out)=0.1V, for 7500N applied,

${1.33 \times 10^{- 5}\frac{V}{N}},$

${13.3\frac{m\; V}{k\; N}},$(59 mV/klbf)

where V_(in)=9V, G=18, R₁=43.8 kOhm, R₂=33.6 kOhm, r=500 μm, t=100 μm,E_(SI)=170 GPa, E_(ST)=200 GPa, A_(SI)=1.1×10⁻⁵ m², A_(ST)=2.4×10⁴ m²for the calculations above.

Sensors, as shown in FIGS. 5-11, were loaded quasi-statically incompression and sensor response curves as a function of applied loadwere generated. The sensor response calibration from the test data wasused in dynamic tests using a Split-Hopkinson bar test apparatus todetermine measured load during high load rate testing. In addition,maximum load at failure was determined experimentally.

FIG. 15 presents chart 1500, representing testing data for quasi-staticcompressive loads for a number of sensing elements. FIG. 15, plots1510-1550 represent time vs. V_(out), for a plurality of load inputs,1000 lbf (4445 N) as shown in plot 1510, 2000 lbf (8890 N) as shown inplot 1520, 3000 lbf (13335 N) as shown in plot 1530, 4000 lbf (17780 N)as shown in plot 1540, and 5000 lbf (22230 N) as shown in plot 1550.

As shown in respective plots 1510-1550, the load inputs ramp up linearlyto the respective maximum level (e.g., 1000 to 5000 lbf) in 1 to 5seconds. Further, as shown, after reaching a defined maximum, the loadsreturn back to zero load in a symmetrical manner. The measured sensorresponse(s) was repeatable. At loading and unloading rates of between 10to 100,000 lbf/sec (44.45 to 4.445×105 N/sec) almost identical responsecurves result. The loading rates presented in FIG. 15 correspond to aminimum signal period of 10 msec and bandwidth of ˜100 Hz, which can beconsidered to be equivalent to a quasi-static test condition.

The presented data was measured for two of the four sensors (e.g., twoof four sensor assemblies 300) located on an epoxy washer 630, whereeach upper line in the load pairing is the load measured on a firstsensor assembly 300, and where each lower line in the load pairing isthe load measured on a second sensor assembly 300. For the arrangementshown in FIG. 6, all four sensor assemblies 300 showed similar responsesto the two presented in FIG. 15, but with different offset voltages. Asshown in FIG. 15, the resulting signals 1510-1550 have a low noise witha S/N ratio of 100:1, facilitating measurement resolution ofapproximately 100 N. As depicted at 1560 (and similarly for all theplots 1510-1550) there is a no-response section of the curve(s) duringwhich the slack in the testing equipment is taken up, and followed by asteeper rise in response at the low load levels as gaps in the sensorpackage 1110 are closed and the full washer area is engaged. Initiallyat low load levels a larger fraction of the load is borne by the Siplate 310, however, once the gaps in the sensor package 1110 arecompressed out the sensor response is approximately linear.

FIG. 16 shows the same data as that presented in FIG. 15 but plotted asload versus sensor output (V), where curves in group 1670 are for thefirst sensor 300 and the curves in group 1680 are for the second sensor300. As shown, hysteresis is small. If the initial low load parts of thecurves are ignored the response is linear with a slope of approximately7 mV/1000 lbf mV (1.57 mV/kN) for both sensing elements 300. The sensingelements 300 depicted were identical designs with a plate diameter of 1mm and a thickness of approximately 100 μm. Calibration curves from thisdata (sensors 1 and 4) are shown in FIGS. 17 and 18 based on a quadraticcurve fit (e.g., 2^(nd) order least squares polynomial fit) and wereused to measure loads during dynamic testing at much higher load rates.FIG. 17, plot 1710 presents the data while plot 1720 is the calibrationcurve (quadratic curve fit). FIG. 18, plot 1810 presents the data whileplot 1820 is the calibration curve (quadratic curve fit).

The test data from the dynamic tests was generated using aSplit-Hopkinson bar apparatus. Briefly, the Split-Hopkinson bar utilizesa gas-gun to drive a projectile into a long steel rod and impart ashaped shock pulse along the rod. A pulse shaper (typically a copperdisk) at the impact point of the projectile on the incident bar isutilized to shape the pulse, imparting, for example, a square wave ortriangle wave pulse. The test sample (e.g., a sensor assembly 600containing the sensing elements 300) is placed at the end of theincident bar opposite the projectile impact. Two configurations weretested, a direct load configuration and a preloaded protected sensorconfiguration in which the sensor assembly 600 is inside the protectedhousing of sensor washer 1110.

The calibration curves from the quasi-static testing were utilized toconvert V_(out) recorded (e.g., on a high speed digital oscilloscope)into force for the data shown in FIG. 19 (a square wave pulse input) andFIG. 20 (triangle wave pulse input). FIG. 19, plot 1910 is the datameasured for the first sensor 300, plot 1920 is the data measured forthe second sensor 300, and plot 1930 is the data measured from aquartz-based calibration load cell. FIG. 20, plot 2010 is the datameasured for the first sensor 300, plot 2020 is the data measured forthe second sensor 300, plot 2030 is the data measured from aquartz-based calibration load cell. As shown in FIGS. 19 and 20, thefirst and second sensors 300 matched the calibration data from the loadcell, plots 1930 and 2030.

A degree of noise is present for the sensor element 300 signals as wellas the quartz load cell. The noise can be due to both electronic andmechanical sources. Ringing and overshoot at the front of the measuredpulse are evident for both the quartz gage and the sensor elements 300.Overshoot is more pronounced for the sensor elements 300. An approach toclean up the sensor elements 300 signals was to apply a low levelpreload (approximately 2 kN) to the sensor washer 1110. Such an approachcan remove any slack internal to the sensor washer 1110 and further,move the sensor elements 300 signals from the quadratic to the linearregion on the calibration curve, as well as reduce mechanical noisesources.

In some of the samples tested, the sensor washer 1110 was still in astate of compression after preload. FIG. 21 presents a chart 2100 for atriangle shaped pulse with the preload. FIG. 21, plot 2110 is the datameasured for the first sensor element 300, plot 2120 is the datameasured for the second sensor element 300, and plot 2130 is the datameasured from the quartz-based calibration load cell.

The noise on signals from the sensor elements 300 was significantlyreduced and the overshoot at the front of the pulse measurement waseliminated. The square wave pulse (as shown in FIG. 20) was similarlycleaned up by preloading. Further, electronic noise sources were notcleaned up, specifically 60 Hz noise, as is present in FIG. 19 or 20. Ifelectronic noise sources were reduced (e.g. with a 60 Hz filter) ameasurement resolution similar to that shown for the quasi-static data(e.g., per FIG. 16) can be achievable. A peak load of 35,000 N (7900lbf) was measured, as shown in FIG. 21, which corresponds to a stress atthe sensor washer of about 140 MPa (20 ksi). Further, elimination of thesquared term in the calibration curve (not shown) brought the preloadedMEMS sensor data to closer agreement with the quartz calibration sensor.A signal rise time of about 10 μsec was tracked by the sensors (FIGS. 18and 19) corresponding to a maximum demonstrated frequency of about100,000 Hz.

The test results indicate the failure load was about 5 times higher thanexpected (>35000 N measured vs 7500 N predicted (P_(App))) and thesensor response was 8 times lower than expected (1.57 mV/kN measured vs13.3 mV/kN predicted). The predicted load failure was based on anoversimplification of the physical situation and hence was purposelyconservative. It was anticipated that the sensor response curve wouldapproximately track the load since the strain and output voltage trackthe stress at the edge E of the plate 310. A key oversimplification forthe previously presented design calculation was the application of afixed/fixed boundary condition. In the sensor package the thick steel indie mount 910 in contact with a sensing element 500 would not allow muchdeformation and would tend to produce a fixed boundary. It is to benoted that in the data shown, e.g., FIGS. 16-21, a thin (0.001 inch,0.0254 mm) layer of polyimide tape (a relatively soft material) wasplaced in between the PCB 530 and the die mount 910 to provide somemeasure of wear protection. This soft material would tend to make theboundary more compliant (less fixed). Also the PCB 530 at the edge ofthe sensor plate contacting the thicker section of the sensing element300 is a softer material that would allow some deformation. Hence, theboundary condition at the edge of the sensor plate may be improved bymodeling as somewhere between a fixed boundary and a simply supportedboundary in which the slope at the edge of the plate is not assumedzero. For a simply supported circular plate the edge stress is zero, soany flexibility at the edge E of the plate 310 would tend to reduce thestress, increase the maximum achievable load, and reduce the signaloutput. For a simply supported plate it would be better to have thepiezoresistive strain sensing elements at the center of the plate ratherthan the edges.

The maximum survivable load may be under-predicted and the sensorresponse may be overpredicted may be due to the effect of the etchfillet radius FR at the edge E of the sense plate 310 is not modeled inthe design calculation previously described. As shown in FIG. 13 a sharpcorner at the edge FR of the sense plate 310 causes a stressconcentration resulting in plate stresses beyond the fracture limit forSi in bending. As previously mentioned, a radius at this location willreduce this stress concentration leading to a higher load required forfracture, less strain, and therefore less sensor response voltage.

As previously mentioned, a micro-preload can be applied to the sensorassembly 1110, for example to compensate for the various componentscomprising assembly 1110 are not tightly packed when assembly 1110 isfirst assembled. Specifically parts of the load path may be in contactwhile other parts are not. An amount of preload can be required tocompress the initially contacted parts of the washer assembly 1110enough to bring the full washer area into contact in order to achievefull load division. In particular the die stickup (FIG. 5, distance M)may be such that the assembly 510 is in contact with the die mount 910while the surrounding PCB 530 is not. The sensor plate 310 can initiallycarry the entire load as the sensor washer 1010 is compressed enoughuntil the PCB 530 comes into contact with the die mount 910. A functionof the sensor plate 310 initially carrying the entire applied load canaccount for the initially higher slope of the load response curves(quadratic term in the calibration equations). In an embodiment, a smallamount of preload can be applied to cause the entire areas of therespective components 910, 530 and 310 to contact and the sensorresponse to enter the linear range. Owing to the ability to monitorsensor response voltage during the preload process (per FIGS. 17 and 18)it is possible to determine how much preload is required to compress thevarious components comprising sensor assembly 500 and thus determinewhen enough micro-preload has been applied for complete contact.

As previously described (e.g., per Eqn. 1) the sensor survival load ismost sensitive to a thickness of thinned region 312 versus platethickness 310, and further a radius J of the thinned region 312, perFIG. 3. Reducing the plate radius J or increasing the thickness ofthinned region 312 can result in significantly higher maximum achievableloads (squared functionality for both parameters). However, such anapproach can lead to a loss of sensor response (per. Eqn 18). Inaddition, reducing the percentage of load carried by the sensor plate310 and thinned region 312 by decreasing the size of the die mount 910and increasing the stiffness of the surrounding materials (e.g.Tungsten, E=400 GPa, instead of Steel, E=200 GPa) can increase themaximum achievable load before sensor plate failure, but with a linearfunctionality (per Eqns. 1, 13 and 14).

An approach to increase sensor response for a given strain is toincrease the number of strain-sensitive ‘wires’ (e.g., traces 320 and330) on the surface of the thinned sensor region 312 and position thewires more precisely at the maximum strain locations on the plate 310.For instance, for a calculation to accurately determine the platedeformation in terms of fixed vs. simply supported boundary conditions,an approach may be to place the highest density of ‘wires’ more at thecenter of the thinned region 312 than at the edge E. The number of wirescan be more densely patterned than that shown in FIG. 3 owing to thetraces being photolithographically patterned much thinner and closertogether in order to increase R₁ and R₂ (e.g., as defined in Eqn. 18)and thereby increase sensor output voltage for a given load. Hence, inan embodiment, to achieve a more sensitive, higher maximum load sensorcan involve increasing the radius J to provide more area forpiezoresistive ‘wires’ (a denser pattern of traces at the optimum radiallocation micromachined at the limit of photolithographic precision),while at the same time increasing the thickness of the thinned region312 such that the plate 310 can handle a higher load.

FIG. 22 presents a sensor apparatus 2200, in accord with an embodiment.As shown, the load sensor 120 can be placed where the washer 150 islocated in FIGS. 1 and 2. By locating the load sensor 120 between thebolthead 131 and the support structure 110, a compressive load appliedto the outer surface 134 of bolthead 131 can be measured directly atsensor 120. It is to be appreciated that while the various embodimentspresented herein are directed towards an arrangement whereby a loadsensor 120 is located on a boltshaft 133 of a bolt 130, the variousembodiments are not so limited and can be directed towards anyconfiguration which can utilize at least one Si sensing element andassociated structure(s) as presented herein. For example, the sensingelements 300 can be incorporated into a system whereby the sensingelement(s) 300 are positioned between two plate structures whichmaintain a compressive load on the sensing element(s) 300 duringoperation.

FIGS. 23 and 24 illustrate exemplary methodologies relating to a sensorwhich can measure and survive shock loading while still providing a highsignal/noise ratio with significant bandwidth are presented. While themethodologies are shown and described as being a series of acts that areperformed in a sequence, it is to be understood and appreciated that themethodologies are not limited by the order of the sequence. For example,some acts can occur in a different order than what is described herein.In addition, an act can occur concurrently with another act. Further, insome instances, not all acts may be required to implement themethodologies described herein.

FIG. 23 illustrates a methodology 2300 relating to a sensor which issubjected to a compressive preload and can measure and survive shockloading while still providing a high signal/noise ratio with significantbandwidth are presented, according to an embodiment. As previouslymentioned, a preload can be applied to the sensor, whereby duringoperation of the sensor, the magnitude of the preload can be reduced asa function of the applied operational load. With knowledge of thepreload amount, and the final preload during sensor operation, themagnitude of the applied operational load can be determined.

At 2310, a load sensor can be formed and include at least onepiezoresistive sensing element. As previously described, the sensingelement can be formed from Si. A concern with Si elements is theirinherent brittleness in the presence of a tensile stress, a concernwhich can be addressed by application of a compressive preload on the Sisensing element. Based, at least in part, upon the application, thesensing element can be placed in a containing structure which canprotect the sensing element(s) as well as also containing other materialcomponents which can minimize impact stresses that could cause fractureof the sensing element. In an embodiment, the load sensor can beutilized in a configuration where the load sensor is supported on aboltshaft, where the load sensor is in the form of a washer. Tofacilitate determination of application of loading in a number ofdirections (e.g., off-center loading) relative to the load sensor, theload sensor can be formed with a plurality of sensing elements, e.g.,four sensing elements to facilitate load determination in fourdirections (e.g., effectively cardinal directions) relative to theposition of the load sensor on the boltshaft. The containing structurecan be formed from steel. Further, the material components can be placedeither side of the sensing element(s) to provide a degree of complianceto the load sensor. The materials can be formed from any ductile and/orcompliant material, as previously described herein.

At 2320, a micro-preload can be applied to the load sensor. Themicro-preload can act to compensate for the various components in theload sensor not being tightly packed during assembly of the load sensor,e.g., parts of a load path may be in contact while other parts are not.As previously described, an amount of preload can be applied to compressinitially contacted parts of the load sensor to achieve full loaddivision between the plurality of sensing elements, and hence facilitatea linear sensing response.

At 2330, as previously described particularly with reference to FIGS. 1and 2, the load sensor can be located on a support structure which canbe utilized to locate the load sensor relative to a component to besensed, and also to apply a macro-preload to the load sensor. A supportstructure can be a nut and bolt, onto which the load sensor is located,whereby the bolt is located in a recessed portion of the component to besensed, e.g., a countersunk hole in a wall of the component to besensed. A plurality of spacing components, e.g., washers, can beutilized to facilitate load absorption during operation of the loadsensor. The washers can be formed from ductile and/or compliantmaterial.

At 2340, a macro-preload can be applied to the sensor, where, in anembodiment, the macro-preload can be of a magnitude greater than ananticipated maximum load to be experienced during operation of the loadsensor. The macro-preload can be several times greater than themicro-preload. For example, with the exemplary embodiment, a compressivemacro-load can be applied to the load sensor, e.g., by tightening of thenut on the bolt, where the loading results in tensile stress beingapplied to the boltshaft, the tensile stress can be equal but ofopposite magnitude to the compressive macro-load.

At 2350, during operation a load can be applied to the bolthead whichcan act to force the boltshaft in an inward direction relative to thecomponent wall. Accordingly, owing to the boltshaft moving in adirection opposite to the loading direction of the nut and bolt, thecompressive load resulting from the compressive macro-preload isreduced.

At 2360, the amount by which the compressive preload reduced can bedetermined, thereby enabling determination of the magnitude of the loadapplied to the bolthead.

FIG. 24 illustrates a methodology 2400 relating to fabricating a sensorwith a thinned region to facilitate concentrating a load at the thinnedregion. At 2410, a plate can be formed to act as a sensing plate. In anembodiment, the plate can be formed from Si. In another embodiment, theplate can have a thickness of about 100 μm.

At 2420, a portion of the sensing plate can be thinned. The thinnedregion can be of any suitable thickness, e.g., about 10 μm thick, tofacilitate a higher degree of strain occurring in the thinned regionduring loading than occurs in the un-thinned portion of the plate. Thethinned region can be formed by any suitable operation, such as anetching operation. Further the thinned region can have a circularconfiguration.

At 2340, at least one trace element can be formed over the thinnedregion. The trace(s) can form a Wheatstone bridge, or similar circuitsystem, where, in an embodiment, the trace(s) can be of a circularserpentine pattern configured to operate over the thinned region.Appropriate connections, wiring, etc., can be attached to the trace(s).The sensing plate with the traces formed thereon can be located in aload sensor, as previously described.

At 2440, the load sensor can be incorporated into a load sensing system.During application of a load to the load sensor, a strain can be createdat the thinned region owing, in part, to the difference in thicknessbetween the thinned region and the rest of the thicker plate. The strainat the thinned region can further induce strain in the trace(s) whichcan accordingly affect the voltage, etc., across the Wheatstone bridge.Based upon the change in voltage on the Wheatstone bridge the reductionin compressive load on the plate can be determined and accordingly, thedegree of applied load.

What has been described above includes examples of one or moreembodiments. It is, of course, not possible to describe everyconceivable modification and alteration of the above structures ormethodologies for purposes of describing the aforementioned aspects, butone of ordinary skill in the art can recognize that many furthermodifications and permutations of various aspects are possible.Accordingly, the described aspects are intended to embrace all suchalterations, modifications, and variations that fall within the spiritand scope of the appended claims. Furthermore, to the extent that theterm “includes” is used in either the details description or the claims,such term is intended to be inclusive in a manner similar to the term“comprising” as “comprising” is interpreted when employed as atransitional word in a claim.

The invention claimed is:
 1. A method, comprising: applying acompressive preload to a piezoresistive device, wherein the compressivepreload is greater in magnitude than an operating range of operationalloads; transmitting an operational load within the operating range tothe piezoresistive device so as to produce a reduction in thecompressive preload; and measuring the reduction in the compressivepreload to facilitate determining a magnitude of the operational load.2. The method of claim 1, wherein the compressive preload is applied tothe piezoresistive device by tightening of a nut on a boltshaft of abolt, wherein the bolt is located on a supporting structure to which theoperational load is applied.
 3. The method of claim 2, wherein abolthead of the bolt is located in a hole extending from an outersurface of the supporting structure to an inner surface of thesupporting structure, and the piezoresistive device is located on theboltshaft between the nut and the inner surface of the supportingstructure, the tightening of the nut applying the compressive preload tothe piezoresistive device.
 4. The method of claim 1, wherein thepiezoresistive device is a plate fabricated with a thinned inner regionand a thicker outer region, the thinned inner region facilitatesinducing strain into a conductive trace element located at the thinnedinner region facilitating the measuring of the reduction in compressivepreload.
 5. The method of claim 1, wherein the piezoresistive devicecomprising a plate of silicon.
 6. A method, comprising: applying acompressive preload to a piezoresistive device, wherein the compressivepreload is greater in magnitude than an operating range of operationalloads; transmitting an operational load within the operating range tothe piezoresistive device; and measuring the reduction in thecompressive preload.
 7. The method of claim 6, wherein the compressivepreload is applied to the piezoresistive device by tightening of a nuton a boltshaft of a bolt, wherein the bolt is located on a supportingstructure to which the operational load is applied.
 8. The method ofclaim 7, wherein a bolthead of the bolt is located in a hole extendingfrom an outer surface of the supporting structure to an inner surface ofthe supporting structure, and the piezoresistive device is located onthe boltshaft between the nut and the inner surface of the supportingstructure, the tightening of the nut applying the compressive preload tothe piezoresistive device.
 9. The method of claim 6, wherein thepiezoresistive device is a plate fabricated with a thinned inner regionand a thicker outer region, the thinned inner region facilitatesinducing strain into a conductive trace element located at the thinnedinner region facilitating the measuring of the reduction in compressivepreload.
 10. The method of claim 6, wherein the piezoresistive devicecomprising a plate of silicon.
 11. A method, comprising: applying acompressive preload to a piezoresistive device, wherein the compressivepreload is greater in magnitude than an operating range of operationalloads; transmitting an operational load within the operating range tothe piezoresistive device; and measuring the reduction in thecompressive preload to facilitate determining a magnitude of theoperational load.
 12. The method of claim 11, wherein the compressivepreload is applied to the piezoresistive device by tightening of a nuton a boltshaft of a bolt, wherein the bolt is located on a supportingstructure to which the operational load is applied.
 13. The method ofclaim 12, wherein a bolthead of the bolt is located in a hole extendingfrom an outer surface of the supporting structure to an inner surface ofthe supporting structure, and the piezoresistive device is located onthe boltshaft between the nut and the inner surface of the supportingstructure, the tightening of the nut applying the compressive preload tothe piezoresistive device.
 14. The method of claim 11, wherein thepiezoresistive device is a plate fabricated with a thinned inner regionand a thicker outer region, the thinned inner region facilitatesinducing strain into a conductive trace element located at the thinnedinner region facilitating the measuring of the reduction in compressivepreload.
 15. The method of claim 11, wherein the piezoresistive devicecomprising a plate of silicon.